Whether long-chain omega-3 polyunsaturated fatty acids (PUFAs) reduce the risk of primary cardiac arrest, also known as sudden cardiac death, remains unknown. We propose to conduct a community-based case-control study to explore the relation of dietary longchain omega-3 PUFA intake, red-cell membrane omega-3 PUFA levels, and the risk of primary cardiac arrest (PCA). We will identify through Emergency Medical Service incident reports all cases of PCA, aged 25-74 years, without prior clinical heart disease, occurring in Seattle and suburban King County, Washington, during the period April 1990 to March 1994. Control subjects will be randomly identified from the same community using random digit dialing. Blood specimens from cases (collected by paramedics in the field within 45 minutes of PCA) and from control subjects will be analyzed to determine red-cell membrane fatty acid composition. (Pilot data from primates suggest that this marker of recent PUFA intake is not likely to be altered by the occurrence of cardiac arrest and resuscitation.) Spouses of cases and controls will be interviewed to assess dietary intake of long-chain omega-3 PUFAs (a function of the amount of seafood consumed), usual fat consumption (based upon the Northwest Lipid Research Clinic Fat Index), changes in dietary intake, and the presence or absence of other risk factors for PCA. Analyses will be conducted to assess the relation between dietary intake and red-cell membrane omega-3 fatty acid levels (eicosapentaenoic and docosahexaenoic acid) and the risk of PCA. We will determine whether dietary omega-3 PUFA intake and red-cell membrane omega-3 PUFA levels are associated with a reduced risk of PCA. For each fatty acid, we also will determine if there is a threshold and/or dose response effect. A combination of the unique setting and the collaboration of investigators from a variety of relevant disciplines provide a unique opportunity to examine the relation of dietary long-chain omega-3 PUFA intake, red-cell membrane omega 3 PUFA levels, and the risk of PCA.